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GEO 200

ICA 5: Atmosphere, Energy, and Temperature

Atmospheric Composition_

Depending on height above the surface, the atmosphere has a different chemical composition. There are two distinctly different regions within the larger atmosphere that we call the heterosphere and the homosphere. The homosphere is the region that is closest to the surface and because this region experiences turbulence, it is composed of a well-mixed layer of gases that include nitrogen (N₂), Oxygen (O₂), Argon, (Ar), carbon dioxide (CO₂), and trace amounts of various other gases. This region exists from the surface up to approximately 50 mi above the surface. Above this region is the heterosphere, which does not experience turbulence. Thus, the gases within the heterosphere separate by atomic weight. First there is a region of nitrogen (N₂) that extends to approximately 150 mi above the surface, then a region of atomic oxygen (O) that extends to approximately 600 mi, then a region of helium (He) that extends to approximately 1500 mi, and finally a region of hydrogen atoms (H) that extends to over 6000 mi.

Use the information in the previous paragraph to complete the following:

1. Draw a line on the given atmospheric column that indicates the division between the heterosphere and the homosphere.
2. Shade the homosphere with any color.
3. In the section that represents the heterosphere, draw lines parallel to the surface that indicate the divisions between its sub regions of lighter gases.
4. Label each gas sub region in the Heterosphere with the name of its gas and the appropriate chemical symbol. Shade each of these regions with a different color.

Refer to the thermal structure of the atmosphere depicted above, and compare it with your sketch of the gas regions in the atmosphere to answer the following questions:

1. How much of the Troposphere is contained within the homosphere?
2. Which gases would be found within the Mesosphere?
3. Would the gases in the Stratosphere be well mixed or separated into distinct layers?
4. Would the gases in the Thermosphere be well mixed or separated into distinct layers?

Radiation and the Greenhouse Effect

Energy from the Sun enters Earth’s atmosphere in the form of shortwave radiation (insolation). The average amount of energy received at the top of the atmosphere is known as the solar constant, and is approximately 1373 W/m² (watts per square meter). Upon entering the atmosphere, approximately 26% of the insolation is either immediately reflected back to space or scattered by particles in the air; an additional 19% of the insolation is absorbed by water molecules in the atmosphere (clouds).

After traveling in the atmosphere, another 4% of the incoming shortwave radiation is reflected by the Earth’s surface. The proportion of solar radiation reflected by a surface is the albedo, which is controlled by surface color and texture. Light-colored, smooth surfaces (high albedo) reflect a larger proportion of insolation, while dark-colored, rough surfaces (low albedo) reflect smaller proportions.

Of the 1373 W/m² of solar energy that enters the top of the atmosphere, an average of 51% is finally absorbed by the surface of the Earth. It is this energy that is available for heating the ground and air.

While the Earth absorbs shortwave radiation from the Sun, it emits energy in the form of longwave radiation. Clouds, water vapor, and other greenhouse gases in the atmosphere absorb the longwave radiation emitted by the Earth’s surface. The clouds and greenhouse gases then reradiate longwave energy back to the surface, preventing the loss of all longwave radiation to space. This naturally occurring process, the greenhouse effect, serves to keep the Earth’s surface warm and habitable. Global warming is the concern that an increase in greenhouse gases is intensifying the natural greenhouse effect to the detriment of society.

2. Recall that of the 1373 W/m² solar energy that enters the top of the atmosphere, only 51% reaches Earth’s surface. Express this value in W/m². What is 51% of 1373 W/m²?
3. If the Earth experienced an increase in average albedo from 4% to 8%…
   1. Would this increase or decrease the solar radiation that reaches the surface?
4. Calculate the amount of solar energy that would reach Earth’s surface, expressed in W/m².
5. List two natural phenomena that might cause the Earth’s albedo to increase (Note: “Global warming” or “the Greenhouse Effect” are NOT correct responses).
6. How would an increase in the Earth’s albedo likely affect the climate of the Earth?
7. Describe surface air temperatures on cloudy nights.
8. Describe surface air temperatures on clear nights.

Insolation and Temperature

The atmosphere is primarily transparent to shortwave radiation and reactive to longwave radiation. The Earth’s surface emits longwave radiation that slowly warms the atmosphere above the surface of the Earth, and in turn, the lower atmosphere helps to reheat the Earth’s surface. Thus, the relationship between insolation and air temperature is not direct; there is a lag effect. The warmest time of day occurs at the moment of maximum longwave radiation emitted to the atmosphere from the ground, and not at the moment of maximum insolation.

Image: The typical diurnal cycle of surface temperature and the net energy rate due to incoming solar and outgoing longwave radiation.

Encanto Golf Course is located in central Phoenix, AZ. The table below lists insolation and temperature readings taken at Encanto Golf Course on a summer day.

Table: Insolation and Air Temperature Observations for Encanto Golf Course on June 26, 1990.

Answer the following questions using the table.

7. Why is the insolation zero during some hours?
8. List the time of the maximum air temperature. List the time of the maximum solar radiation.
9. List the time of the minimum air temperature. List the time range of the minimum solar radiation
10. Notice there is a “lag” or delay between the receipt of maximum solar radiation and the maximum temperature; there is also a lag or delay between the minimum solar radiation and the minimum temperature. Why is there a delay? Explain.

Elevation and Temperature

Within the troposphere, temperatures decrease with increasing altitude above the Earth’s surface: the normal lapse rate of temperature change with altitude is 6.4 C°/1000 m, or 3.5 F°/1000 ft. Worldwide, mountainous areas experience lower temperatures than do regions near sea level, even at similar latitudes. The consequences are that average air temperatures at higher elevations are lower, nighttime cooling increases, and the temperature range between day and night and between areas of sunlight and shadow also increases. Temperatures may decrease noticeably in the shadows and shortly after sunset. Surfaces both gain heat rapidly and lose heat rapidly due to the thinner atmosphere.

Temperature data are presented in Table 8.1 for La Paz and Concepción. Both stations are approximately the same distance south of the equator but differ in elevation. La Paz is at 4103 m (13,461 ft), whereas Concepción is at 490 m (1,608 ft) above sea level. The hot, humid climate of Concepción at its much lower elevation stands in contrast to the cool, dry climate of highland La Paz.

People living around high-elevation La Paz actually grow wheat, barley, and potatoes – crops characteristically grown in the cooler midlatitudes at lower elevations. These crops do well despite the fact that La Paz is 4,103 m above sea level. The combination of elevation (moderating temperatures) and equatorial location (producing higher Sun altitude and consistent daylength) guarantee La Paz these temperature conditions, averaging about 9 C° (48 F°) for every month. Such moderate temperature and moisture conditions lead to the formation of more fertile soils than those found in the warmer, wetter climate of Concepción.

11. Since we want to compare temperatures at different elevations; why does it matter that we select two locations at a similar latitude?

12. Using the temperature graphs provided, plot the data from Table 8.1 for these two cities. Use a smooth curved line graph to portray the temperature data.
13. Calculate the average annual temperature for each city (to calculate: (maximum + minimum)/2).
14. Calculate the annual temperature range for each city (maximum – minimum).
15. Why are the temperatures at La Paz more moderate in every month and so consistent overall as compared to Concepción?
16. The annual march of the seasons and the passage of the subsolar point between the Tropics of Cancer and Capricorn affect these stations. Can you detect from your temperature graphs these seasonal effects? Explain.

Marine vs. Continental Effects

The irregular arrangement of landmasses and water bodies on Earth contributes to the overall pattern of temperature. The physical nature of the substances themselves—rock and soil versus water—is the reason for these land-water heating differences. More moderate temperature patterns are associated with water bodies compared to more extreme temperatures inland.

These contrasts in temperatures are the result of the land-water temperature controls: evaporation, transmissibility, specific heat, movement, ocean currents, and sea-surface temperatures. The term marine, or maritime, is used to describe locations that exhibit the moderating influence of the ocean, usually along coastlines or on islands. Continentality refers to the condition of areas that are less affected by the sea and therefore have a greater range between maximum and minimum temperatures diurnally (daily) and yearly.

The Canadian cities of Vancouver, British Columbia, and Winnipeg, Manitoba, exemplify these marine and continental conditions. Both cities are at approximately 49°N latitude. Respectively, they are at sea level and 248 m (814 ft) elevation. However, Vancouver has a more moderate pattern of average maximum and minimum temperatures than Winnipeg. Vancouver’s annual range of 11.1 C° (20.0 F°) is far below the 38.8 C° (70.0 F°) temperature range in Winnipeg. In fact, Winnipeg’s continental temperature pattern is more extreme in every aspect than that of maritime Vancouver.

17. Using the data given in Table 8.2, plot the temperatures for these two cities and portray with a smooth curved line graph on the temperature graph in Figure 8.3. Calculate the average annual temperature and temperature range for each city.

Name _________________________________________________

In-class activity 4: Earth – Sun Relationships

Introduction

Solar radiation that enters the Earth-Atmosphere system is the primary source of energy for nearly every atmospheric process on Earth.  The unique relationship between the Earth and Sun is what causes the seasons, controls the length of days, and organizes the basis for keeping track of time.  An understanding of this relationship is essential when learning about atmospheric processes on Earth.

Basic Earth-Sun Geometric Relationships

The earth’s orbit around the sun is elliptical, varying the distance between the earth and sun throughout the year.  While the average distance between the earth and the sun is approximately 150 million kilometers (93 million miles), the actual distance at any given time fluctuates by as much as 5 million kilometers (3 million miles).  The earth is nearest the sun (perihelion) during the Northern Hemisphere’s winter (January) and is farthest from the sun (aphelion) during the Northern Hemisphere’s summer (July).

Seasons occur due to the tilt of Earth’s axis of rotation.  The axis is an imaginary line that connects both poles, and it is tilted at an angle of 23.5° relative to the plane of the ecliptic, the plane on which the Earth revolves around the Sun.  Since the axis of rotation is always oriented in the same direction (pointing toward the North Star), different latitudes receive direct solar radiation at different times throughout the year.

Due to its rotation, half of the Earth is always receiving some portion of Sunlight, known as the circle of illumination.  However, the tilt of the Earth’s axis also controls daylength.  During June, the Northern Hemisphere is tilted toward the Sun and experiences longer daylengths.  During December, the Northern Hemisphere is tilted away from the Sun and experiences shorter daylengths.

Solar Declination

The seasonal temperature changes are controlled by the amount of direct radiation received at the surface.  As a result of the tilt of the axis and the curvature of the Earth, some latitudes receive direct radiation while other latitudes receive radiation at an oblique angle.  When radiation strikes an object at an oblique angle, the energy is distributed over a larger area and is less intense.

The latitude at which the Sun is directly overhead at noon is the solar declination.  The solar declination for the June Solstice is 23.5°N (Tropic of Cancer), and 23.5°S (Tropic of Capricorn) for the December Solstice.  During both Equinoxes, the solar declination is at the Equator (0°).  The solar declination changes every day as the Earth revolves around the Sun, but is constrained between the Tropics.

1. List the date and the solar declination for each position.

2. Label the diagram below with the appropriate date for each position.

3. In the diagram below, what is the date?

4. Using the diagram above, describe the day or night length from the Arctic Circle to the North Pole.

5. What percentage of the Earth is illuminated at noon December 21 (or at any time)?

6. How many hours of daylight does the South Pole receive on March 21?

7. How many hours of daylight does the South Pole receive on June 21?

8. Which latitude(s) experience the GREATEST seasonal change in daylight hours? (In other words, do any areas on the globe change from completely dark to completely lit over the year? Where does this happen?)

9. What would happen if the earth’s axis of rotation was NOT tilted at a 23.5° angle?

10. Give the numerical latitude and cardinal direction for the 5 major lines of latitude.

Solar Angle

In addition to the solar declination, it is useful to understand some related geometric terms:  zenith angle: the angle between a point directly overhead and the Sun at solar noon, and solar angle: the angle of the Sun above the horizon at solar noon.  These angles are important because they determine the amount of insolation (incoming solar radiation) potentially received at the surface of the Earth.

To determine the zenith angle at a particular location, calculate the number of degrees of latitude separating the solar declination and the location in question.  If the declination or latitude is in the southern hemisphere, it will be a negative value.  The zenith angle should always be positive; therefore, you should report the absolute value of the zenith angle.

Example:  zenith angle = (location latitude) – (solar declination)

At Alexandria, VA (39°N) on January 20 (solar declination: 20°S)

Zenith angle = 39° – (-20°)

Zenith angle = 59°

At Sao Paulo, Brazil (23°S) on January 20 (solar declination: 20°S)

Zenith angle = -23° – (-20°)

Zenith angle = -3°

Absolute value zenith angle = 3°

The solar altitude angle is calculated by subtracting the absolute value of the zenith angle from 90°.  As the solar declination progresses, the zenith angle decreases and the solar altitude increases.  At solar noon at the latitude of the solar declination, the zenith angle is 0° and the solar altitude angle is 90°.  The zenith angle and the solar altitude angle are significant because the Sun’s rays are much more intense where they strike the Earth directly (zenith angle of 0° and a solar altitude of 90°) (Figure 3.3).

Figure 3.3:  Zenith angle (A) and solar altitude angle (B) for 30°N on December 21.

11. First, calculate the zenith angle for Alexandria, VA (39°N), St. Petersburg, Russia (60°N), and Sydney, Australia (33°S) on the following dates. Show your work, and then check your work before you proceed with the solar angle table.

12. Now, using your answers from the table of zenith angles, calculate the solar angle for Alexandria, VA (39°N), St. Petersburg, Russia (60°N), and Sydney, Australia (33°S) on the following dates. Show your work.

13. Graph your solar altitude angle results for Alexandria, St. Petersburg, and Sydney on a line graph. Your x-axis should be time of year, and your y-axis should be solar altitude angle.  A line graph requires that you connect the plotted data with a line, per each location, so you will have 3 different lines.  Make sure that you follow the rules of making graphs and supply a name for the graph, and correct units and labels for each axis.

Answer the following questions using your graph.

14. Which location likely receives the most insolation during June?

15. Which location is probably the warmest during December?

16. In which month does Sydney likely receive the most insolation?